Sprag-type freewheel clutches are used as one-way clutches in automatic transmissions or in conveyors, for example. They transmit torsional loads in one direction only, the so-called locking direction. In the other direction, the idling direction, sprag-type freewheel clutches move freely. In order to achieve such an effect, sprag-type freewheel clutches comprise an inner race and an outer race, which are concentric with one another and by virtue of their spacing thereby define a gap size H, and sprags, which are arranged so that they can tilt between the races.
On their wedging surfaces the sprags have a convex wedging face. These faces are designed in such a way that a variation of the tilting angle produces a variation of the sprag height in a radial direction. In order to initiate an engagement sequence, the sprags are pressed, by means of springs, for example, against the wedging tracks, which are defined by the races. The direction of the tilting movement depends on the relative movement of the two races. Thus the relative movement in one direction leads to a tilting movement, which reduces the radial sprag height and hence the friction (idling direction), and in the other direction leads to a tilting movement, which increases the radial sprag height, so that a frictional connection is established and torques can be transmitted.
Given a suitably selected center of mass, at a specific relative angular velocity in the idling direction, the centrifugal force acting on the sprags due to the moments of inertia created by the asymmetrical geometry increases to a magnitude which causes the sprags to pivot about an axis, so that a clearance occurs between the sprags and the outer race. A wear-free idling operation is thereby ensured. Such a sprag-type freewheel clutch is disclosed by U.S. Pat. No. 2,824,635, for example.
In this case differing requirements are set for the sprags, depending on the particular application. They are intended to have a low mass, in order that they will have a low mass moment of inertia, and at the same time they should possess a high degree of stiffness. A long wedging contour is advantageous, in order to be able to cover the largest possible tolerances between a shaft and a bore, with a simultaneously low level of stress. Sprags should furthermore have a high tipping load and great freedom of movement in the cage.
In order to increase the stiffness, the sprags, according to DE 1 450 158, for example, are designed so that their curved sections, extending as a mirror image of one another and braced against the outer and inner races, derive laterally inverted from a circular arc section with an adjoining logarithmic curve section. In this case the circular arc sections represent between 3/10 and 5/10 of the overall curved sections. The circular arc sections in the first part serve for the freewheeling state and the remaining part and the logarithmic curve section produce the locking. At the same time the logarithmic curve section is designed so that the angle between the normals at the point of contact and the radius vector is constant over the whole logarithmic curve section, so that as the sprag rolls the engagement angles remain the same. Sprag tilting movements due to the eccentricity of a race thereby have little effect on the engagement angle.
The sprag disclosed in DE 1 450 158, however, has a large mass. In addition, a circular arc component is needed for the freewheel range; the engagement angle varies in this range. Furthermore it is designed for a specific pairing of an inner and an outer race. If the gap size H is different in another application, the specific geometric shape of the sprag means that it is not sufficient simply to scale this, because the stiffness varies as a function of its position, that is to say its orientation in a radial direction.